While the methods, compositions, and materials presented herein can be used in a variety of applications, they are well adapted for use with respect to orthopedic support materials, such as casting tapes and splints. Therefore, as background, characterizations of technology relating to orthopedic casting tapes and problems overcome by the present invention are provided.
Orthopedic casting tapes have been produced using curable resins coated or impregnated onto a substrate. Typically, the casting tape is stored as a roll in a water-impermeable storage pouch until needed for use. When needed, the roll of tape is removed from the pouch and contacted with water. Generally, the tape includes a resin therein that is water-curable. Thus, shortly after the tape is dipped in water, the resin will begin to cure and the tape harden. A medical practitioner applies the casting tape to a patient immediately after it has been soaked with water. Generally, resin materials are chosen that begin to cure immediately after being dipped in water, and that will set, i.e., harden sufficiently to resist passive motion in a joint, e.g., a wrist or ankle, in about 3-5 minutes, and to be weight bearing within about 30 minutes.
Typical resin coatings utilized in conventional orthopedic casting systems include isocyanate-functional polyurethane prepolymers. Generally the resin coatings also include lubricants to facilitate unrolling, application, and molding without the resin interfering. Furthermore, they often include defoaming agents to maintain porosity while carbon dioxide is evolved during the curing process. When exposed to water, the isocyanate functional polyurethane prepolymers cure to form polyureas. In certain situations, it may be desirable to avoid or reduce the level of such reactive groups and their products.
Typical substrates upon which the above resin coatings are applied to produce an orthopedic casting tape are knit fabrics prepared from glass and/or synthetic fibers. Fiberglass or other high modulus fibers contribute significant strength to the cured resin/substrate composite as well as provide a reservoir for the resin during storage and end-use application of the casting tape. Nonglass low modulus substrates generally provide for a lower weight and more radiolucent cast. With most nonglass substrates the strength is limited by the amount of resin that can be held by the substrate. High resin loading with materials known to date must generally be avoided, however, to prevent excessive heat build-up in the cast while it is in its early stages of cure, i.e., during hardening, as a result of the exotherm produced in the above resins.
Typical orthopedic casting tapes are described in U.S. Pat. Nos. 4,667,661 and 4,774,937 (Scholz et al.), which are owned by Minnesota Mining and Manufacturing Company (3M Company) of St. Paul, Minn., the assignee of the present invention. Such tapes are also commercially available from 3M Company under the tradenames Scotchcast.RTM. Plus casting tape and Scotchcast.RTM. 3 casting tape. Construction materials of this type are also described in U.S. Pat. Nos. 4,411,262 and 4,570,622 (von Bonin et al.). These materials are one-component systems containing isocyanate or alkoxysilane groups selected from: (1) optionally substituted aminomethyl alkoxysilane, urea, or biuret derivatives; or (2) uretdione isocyanate functional alkoxysilanes.
A challenge with such conventional polyurethane prepolymer chemistry is to make a low viscosity resin with high strength and high lamination capabilities. That is, it can be difficult to overcome the rapid rise in viscosity with increasing functionality of either polyol or isocyanate, or with decreasing molecular weight of the polyol component. This rise in viscosity is likely due to the increase in hydrogen bonding from the increased concentration of urethane groups in combination with increased chain entanglements caused by increasing functionality.
A need exists for a water-curable resin composition that can be used in a wide variety of orthopedic support materials, whether used in combination with a fiberglass substrate or a nonglass low modulus substrate. That is, a need exists for a low viscosity resin composition that can be used in large amounts, i.e., at high resin loads, without excessive heat build-up, and/or perceived health hazards, as well as in smaller amounts.